On the Chemical Composition of Thermally Treated Coal-Tar Pitches

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Energy & Fuels 2001, 15, 214-223

On the Chemical Composition of Thermally Treated Coal-Tar Pitches R. Meneńdez,*,† C. Blanco,† R. Santamarıá,† J. Bermejo,† I. Suelves,‡ A. A. Herod,‡ and R. Kandiyoti‡ Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain, and Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, London SW7 2BY, U.K. Received August 24, 2000. Revised Manuscript Received November 12, 2000

The aim of this work is to study the changes in the chemical composition of a commercial impregnating coal-tar pitch due to the thermal treatment, using whole samples and the corresponding single isotropic phases and mesophases separated by hot filtration. A deeper knowledge of the molecular size and structure of the components present in the samples was achieved by means of size exclusion chromatography (SEC), planar chromatrography (PC), UVfluorescence (UV-F), solid and solution-state 13C NMR and MALDI-mass spectrometry. The results showed that during thermal treatment there is not only a change in the relative amounts of different molecular size compounds but also in the characteristics of the compounds in each SEC fraction which increase in size with increasing treatment. The study of the single isotropic phases and mesophases yielded some interesting results, which were correlated with the different molecular structure of the compounds present in each phase.

1. Introduction Pitch has proved to be an excellent carbon precursor because of its relatively high carbon yield, graphitizability and the wide range of structures and properties of the resultant carbons. However, when pitch is carbonized for the preparation of a carbon material, some porosity is still generated.1 It has been general practice to treat pitch before carbonization in order to reduce the release of volatile material and consequently the development of porosity. The treatment most commonly applied is that which involves the use of temperature in either an inert or an oxidative atmosphere. It is well documented that either type of pitch treatment, or a combination of both of them, gives rise to the polymerization of the pitch components, resulting in an improvement in the density of the carbons.2,3 Studies of the mechanisms involved in pitch thermal treatment (under inert atmosphere) have shown that the distillation of light compounds together with the dehydrogenative polymerization of pitch components were the main processes involved. The polymerization of pitch includes a series of reactions, such as olygomerization, isomerization, molecular rearrangements, rings closure, etc. These result in the formation of planar molecules of increased molecular size, usually accompanied by the * Corresponding author. Tel: (+34) 985 28 08 00. Fax: (+34) 985 29 76 62. E-mail: [email protected]. † Instituto Nacional del Carbo ´ n, CSIC. ‡ Imperial College, University of London. (1) Savage, G. Carbon-Carbon composites; Chapman and May: London, 1984; pp 157-191. (2) Fujiura, R.; Kojima, T.; Kanno, K.; Mochida, I.; Korai, Y. Carbon 1993, 31, 97-102. (3) Meneńdez, R.; Granda, M.; Fernańdez, J. J.; Figueiras, A.; Bermejo, J.; Bonhomme, J.; Belzunce, J. J. Microsc. 1997, 185, 6-156.

development of mesophase.4-6 In the case of the oxidative treatment of pitch (air-blowing) the results are not so easy to explain, and it has been shown that the mechanisms involved in the process depend on the characteristics of the parent pitch and the severity of the experimental conditions.7-9 The effects of both treatments on pitch have been studied mainly by monitoring the changes of properties such as pitch softening point, carbon yield, solubility parameters, etc. Solid-state techniques (FTIR, X-ray diffraction) and other procedures such as iodine-uptake and transient rheology have corroborated variations in aromaticity, structural order or degree of cross-linking between the molecules, but no real chemical evidence of the effects of the treatments has been found.6,10 Such evidence would be of extraordinary interest for the selection of optimum operational conditions for the preparation of carbons and for deciding the best use of pitch-based products. The main difficulties involved in obtaining detailed chemical information arise from the large molecular size of the pitch components, with the consequent lack of sample solubility. The information obtained by most of the chromatographic and spectroscopic techniques is (4) Lewis, I. C. Carbon 1982, 20, 519-529. (5) Walker, P. L.; Marsh, H. Chemistry and Physics of Carbon; Walker, P. L., Thrower, P. A., Eds.; Marcel Dekker, Inc.: New York, 1979; pp 229-286. (6) Blanco, C.; Santamarıá, R.; Bermejo, J.; Meneńdez, R. Carbon 2000, 38, 517-523. (7) Barr, J. B.; Lewis, I. C. Carbon 1978, 16, 439-444. (8) Zeng, S. M.; Maeda, T.; Tomitsu, K.; Mondori, J.; Mochida, I. Carbon 1993, 31, 191-196. (9) Fernańdez, J. J.; Figueiras, A.; Bermejo, J.; Meneńdez, R. Carbon 1995, 33, 295-307. (10) Meneńdez, R.; Fleurot, O.; Blanco, C.; Santamarıá, R.; Bermejo, J.; Edie, D. Carbon 1998, 36, 973-979.

10.1021/ef000191r CCC: $20.00 © 2001 American Chemical Society Published on Web 12/13/2000

Chemical Composition of Thermally Treated Pitches

Energy & Fuels, Vol. 15, No. 1, 2001 215

Table 1. Main Characteristics of the Parent Pitch and the Thermally Treated Pitches sample

TYa

parent C1 C2 C3 C4 C5

78.9 75.0 71.4 66.6 62.0

SPb

CYc

C/Hd

TIe

NMPIf

MCg

C (%)

H (%)

N (%)

S (%)

O (%)

97 149 174 190

34.6 54.0 61.5 65.6 74.6 79.4

1.64 1.82 1.89 1.95 2.04 2.05

20.0 43.9 53.9 57.5 67.5 69.0

4.7 20.7 29.8 34.7 45.6 49.6

0 10 25 37 46 65

92.0 93.1 93.3 93.6 93.6 93.7

5.0 4.3 4.1 4.0 3.8 3.8

0.9 0.9 0.9 0.9 1.1 1.2

0.6 0.5 0.6 0.5 0.4 0.4

1.8 1.2 1.1 1.0 1.0 1.0

a TY, yield of the thermal treatment (wt %). b SP, softening point (°C). c CY, carbon yield (wt %). d C/H, atomic ratio. e TI, tolueneinsoluble content (wt %). f NMPI, N-methyl-2-pyrrolidone-insoluble content (wt %). g MC, mesophase content (vol %).

limited therefore to just part of the sample. Characterization is even more difficult when the pitches have been previously treated. Solid-state techniques, such as solids-probe mass spectrometry, solid-state NMR, FTIR, etc., despite working on the whole sample, face problems of limited volatility (probe mass), applicability (NMR), or merely provide average information which may be useful for comparative purposes but which does not provide a great amount of chemical detail.11 The use of high-resolution techniques, such as TEM, provides important information on the physical characteristics of the structural units formed from the very initial stages of mesophase formation, but no information has been obtained so far on polymerized but still isotropic pitches.12 The development of mesophase during pitch thermal treatment in an inert atmosphere is fully documented in the literature. These studies are mainly concerned with the use of microscopic techniques.13-15 However, less research has been done on the chemical characterization of mesophase possibly due to the above-mentioned problems.16-19 Blanco et al.20,21 have developed a simple procedure for the separation of the mesophase and the coexisting isotropic polymerized phase in thermally treated pitches which offers the possibility of studying both phases separately. In this way, the complexity of the samples is reduced and the changes of the isotropic phase itself can be determined. From solubility tests and carbon yield values, these authors have observed that polymerization not only occurs in the mesophase but also in the isotropic phase (although to a lesser extent).21 However, further chemical studies which could provide a deeper knowledge of the molecular size and structure of the components of the two coexisting phases in the thermally treated pitches, and at the same time, additional information on the mechanisms involved, would be of great interest. (11) Parker, J. E.; Johnson, C. A.; John, P.; Smith, G. P.; Herod, A. A.; Stokes, B.; Kandiyoti, R. Fuel 1993, 72, 1381-1391. (12) Oberlin, A.; Bonnamy, S.; Rouxhet, P. G. Chemistry and Physics of Carbon; Thrower, P. A., Radovic, L. R., Eds.; Marcel Dekker, Inc.: New York, 1999; pp 2-148. (13) Brooks, J. D.; Taylor, G. H. Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker, Inc.: New York, 1968; pp 243286. (14) Marsh, H.; Meneńdez, R. Introduction to Carbon Science; Marsh, H., Ed.; Butterworths: London, 1989; pp 37-73. (15) Santamarıá, R.; Romero, E.; Gomez, C.; Rodrı´guez-Reinoso, F.; Martinez, S.; Martinez, M.; Marsh, H. Carbon 1999, 37, 445-455. (16) Andresen, J. M.; Martin, Y.; Moinelo, S. R.; Maroto-Valer, M. M.; Snape, C. E. Carbon 1998, 36, 1043-1050. (17) Boenik, W.; Haenel, M. W.; Zander, M. Fuel 1990, 69, 12261232. (18) Kershaw, J. R. Fuel 1995, 74, 1104-1107. (19) Greinke, R. A. Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker, Inc.: New York, 1997; pp 2-43. (20) Blanco, C.; Santamarıá, R.; Bermejo, J.; Meneńdez, R. Carbon 1997, 35, 1191-1193. (21) Blanco, C.; Santamarıá, R.; Bermejo, J.; Meneńdez, R. Carbon 2000, 38, 1043-1051.

Recently, the use of strong solvents and temperature in chromatography has partially overcome the problems of the lack of sample solubility.22,23 More specifically, it has been proved that the use of NMP and temperature in size exclusion chromatography, not only increases the solubility of the pitch but also favors a molecular-sizebased separation at the expenses of other side effects such as polarity.24 In our opinion, this technique can be an important tool for monitoring polymerization processes, when samples of the same nature are involved. The aim of this work is to find chemical evidence which provides a deeper knowledge of the effects of thermal treatment on pitch composition and the contribution of both mesophase and polymerized isotropic pitch to the global behavior of the thermally treated pitch. This paper focuses on the study of the molecular size and structure of a series of pitches obtained from a commercial impregnating coal-tar pitch treated at the same temperature (430 °C) and for different time periods (2, 3, 4, 5, and 6 h), both within the whole samples and in the corresponding single isotropic phases and mesophases obtained by hot filtration. Techniques such as size exclusion chromatography (SEC), planar chromatography (PC), UV-fluorescence (UV-F), solidand solution-state 13C NMR and MALDI-mass spectrometry were used as analytical tools. 2. Experimental Section Materials. A commercial impregnating coal-tar pitch (BI5) supplied by Quı´mica del Naloń S. A. was pyrolyzed at 430 °C for 2, 3, 4, 5, and 6 h. The thermal treatment was carried out in a 2-L stainless steel reactor, under continuous stirring and a nitrogen flow of 40 L h-1, following a procedure previously described.7 The resultant products were named C1, C2, C3, C4, and C5, respectively. The main characteristics of the parent pitch and pyrolysis products are shown in Table 1. Mesophase Separation. The separation of the mesophase and the isotropic phase was carried out by filtration of the thermally treated pitches in a pressurized reactor.20,21 The temperature used in the filtration varied between 300 and 350 °C, depending on the pitch. A nitrogen pressure of 0.5 MPa was employed to force the isotropic phase through the filter. Two fractions were obtained from each pitch. The isotropic ones were labeled I1, I2, I3, and I4, and those corresponding to mesophase (the filtration residues) were labeled A1, A2, A3, and A4. The same correlative numbers as for the parent material are used. Pitch C5 was not fractionated due to problems deriving from the large amount of coalesced mesophase present in the pitch and its high softening point.21 (22) Herod, A. A.; Zhang, S. F.; Kandiyoti, R.; Johnson, B. R.; Bartle, K. D. Energy Fuels 1996, 10, 743-750. (23) Johnson, B. R.; Bartle, K. D.; Herod, A. A.; Kandiyoti, R. J. Chromatogr., A 1997, 758, 65-74. (24) Herod, A. A.; Shearman, J.; Lazaro, M. J: Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 1998, 12, 174-182.

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Characterization of the Whole Thermally Treated Pitches and Single Phases. The parent pitch, the resultant thermally treated pitches, and the corresponding fractions obtained by filtration were previously characterized by measuring their softening point, carbon yield, solubility in toluene and NMP, mesophase content (determined by optical microscopy), and degree of crystallinity (using X-ray diffraction).6,21 Their behavior during pyrolysis was monitored by thermogravimetric analysis.20 In this work, all samples were solubilized in NMP. Samples were placed in an ultrasonic bath for about 2 h to achieve maximum solubilization. The NMP soluble fraction of all the samples was then characterized by means of planar chromatography (PC), size exclusion chromatography (SEC), NMR, and UV. fluorescence spectroscopy (UV-F). Moreover, the whole pitches were fractionated by SEC and the single fractions were characterized by UV-F and MALDI-m.s. Planar Chromatography. Whatman chromatographic plates of silica gel (10 × 20 cm) were used with a sequence of three solvents: (i) acetonitrile, (ii) pyridine, and (iii) toluene. Initially, the plates were washed with pyridine to remove any contaminants from the coating materials and then dried. Development tanks were equilibrated for 0.5 h to saturate the vapor phase before inserting the plates. Samples solubilized in NMP were applied to the plates by multiple spotting at one point and dried in the oven under vacuum to ensure the complete removal of NMP. Once the first solvent reached a distance of 2-3 cm, the plates were removed from the tank and dried before using the second solvent. After the final drying, plates were observed under white and UV light. The plates were then immersed in toluene to facilitate the separation of PAH. As no significant differences between the samples were observed, Rf values were not calculated. Size Exclusion Chromatography. SEC was carried out using a polystyrene-polydivinylbenzene column, 30 cm long, 7.5 mm o.d. (Mixed-D, 5 µm particle size; Polymer Laboratories). In this column polystyrene MM standards from 100 up to 200 000 u may be resolved, showing a linear relationship between log10 MM and the elution volume (or time). Larger MM polystyrene standards up to 2 × 106 u elute at shorter times with a different relationship between MM and time, and are classed as excluded from the column porosity. The whole pitches and the two series of fractions were run at 80 °C, with NMP at a flow rate of 0.5 mL min-1. Detection was carried out at several wavelengths between 280 and 450 nm using a Perkin-Elmer LC250 variable wavelength UV-absorption detector and an Applied Biosciences Diode Array detector in series. The eluent was pumped, using a Perkin-Elmer isocratic pump with a maximum pressure of 14 MPa. To avoid uncertainty, deriving from the concentration of the samples, chromatograms were presented in a peak [area]-normalized mode. Samples were fractionated according to the four peaks observed in the chromatograms of the whole pitches: two fractions from the two peaks in the excluded region (between 6.5 and 8.5 min , and 8.5 and 12.0 min) and two fractions from the two peaks detected in the retained region (between 15 and 19.5 min, and 19.5 and 23.0 min, respectively). Because of the small amounts of sample recovered, the fractions were only analyzed by UV-F and MALDI. UV-Fluorescence Spectroscopy. A Perkin-Elmer LS50 luminescence spectrophotometer with a slit width of 2.5 nm was set to scan at 240 nm min-1. Synchronous spectra were acquired at a constant wavelength difference of 20 nm, using a quartz cell with a 1 cm path length, over the 250-800 nm range. The spectrometer featured automatic correction for changes in source intensity as a function of wavelength. Emission, excitation, and synchronous spectra of the samples were obtained in NMP. The spectra are presented in peaknormalized mode to avoid uncertainty from the concentration of samples. Solutions of the whole thermally treated pitches

Mene´ ndez et al. in NMP and the three SEC fractions obtained from each product were characterized. MALDI-MS. A VG-TOFSPEC was used in linear mode with a nitrogen laser (337 nm), a VAX 4000-mass spectrometer (Fisons, Wythenshawe, UK), and a computer-based data system with OPUS software. An ion extraction voltage of 28 kV with maximum laser power was applied to all samples. Dihydroxybenzoic acid (DHB) was used as matrix. DHB was dissolved in NMP and the solution mixed with the sample in NMP and then applied to the target. Solid- and Solution-State 13C NMR. Solid-state MAS 13C NMR spectra were recorded at 75.5 MHz (7.05T) on a Bruker MSL300 spectrometer using a standard Bruker magic angle sample spinning (MAS) probe with a double-bearing rotation mechanism. The samples were studied as polycrystalline powders in zirconia rotors (7 mm external diameter) and MAS frequencies at 5 kH (with stability better than ca. +-5 Hz) were used. The excitation method was single pulse (SPE) with a 120 s delay between scans. Spinning sidebands were observed for the aromatic signal but they did not overlap with the aliphatic signal over the shift range 5 to 50 ppm. Quantitative measurements included the sidebands as part of the aromatic signal. These bands did not overlap with the aliphatic signal, as was shown by CPMAS/TOSS spectra. No carbonyls signal was observed at the shift of about 170 ppm. All spectra were recorded at ambient probe temperature. The 13C chemical shifts are given relative to tetramethylsilane. Solution-state 13C NMR spectra were obtained using a 400 MHz AMX Bruker NMR spectrometer with a C/H dual 5 mm probe. The quantitative 13C spectra were recorded using a 30° pulse gated 1H decoupling sequence with a repetition scan rate of 1.1 s. No relaxation agent was added. Samples were prepared using a mixed solvent consisting of 50% volume of NMP and deuteriochloroform, with approximate quantities of solvent (800 mg), sample (30 mg), and reference compound tetrakis trimethylsilyl silane (10 mg). The samples were run at ambient temperature.

3. Results and Discussion Thermal treatment of the impregnating pitch at 430 °C for times of 2-6 h resulted in the removal of some of the light components of the pitch followed by an increase in dehydrogenative polymerization with soaking time, according to the increase of the values of the usual parameters (SP, CY, TI, and NMPI) and the amount of mesophase formed (10-65 vol %) as shown in Table 1. Characterization of the separated phases obtained by filtration reveals a considerable increase in these parameters in both the isotropic and the anisotropic phase (Table 2). Although some of these parameters might be affected by molecular associations in the anisotropic phase, the increase in CY and insolubilities in the isotropic phases, where molecular associations are insignificant, is probably mainly due to an increase in molecular size. The solid-state 13C NMR analysis applied to the treated samples shows a strong decrease in the aliphatic carbon content which varies from 5.4 for the parent pitch to 2.2% for C5. The aromatic signal of the solution state 13C NMR spectra was very weak but showed extremely broad lines as would expect for large molecules whose signal strength has diminished from BI5 to C5. On the other hand, the results of X-ray diffraction show an increase in the structural order with time of treatment, interlaminar spacing varying from 3.571 Å for the parent pitch to 3.427 Å for C5, in agreement with the amount of mesophase recorded (Table 1). There is



Table 2. Main Characteristics of the Separated Phases sample

FYa

SPb

CYc

C/Hd

TIe

NMPIff

MCg

C (%)

H (%)

N (%)

S (%)

O (%)

I1 I2 I3 I4 A1 A2 A3 A4

77.0 56.0 45.0 26.0 15.5 36.0 47.0 68.6

151 169 178 202

51.6 56.2 58.9 63.2 72.8 74.9 79.5 82.6

1.80 1.85 1.86 1.91 2.05 2.05 2.06 2.10

40.3 45.8 48.1 51.8 64.9 66.7 71.5 74.2

14.1 16.3 19.6 20.8 52.9 53.4 56.3 57.5

0 0 0 0 87 80 80 75

93.9 94.0 94.2 94.5 93.5 94.6 94.6 94.5

4.3 4.2 4.2 4.1 3.8 3.9 3.8 3.8

0.9 0.9 0.9 1.0 1.1 1.0 1.0 1.2

0.5 0.5 0.5 0.4 0.6 0.5 0.4 0.4

1.2 1.1 1.1 1.1 1.3 1.2 1.1 1.1

a FY, filtration yield (wt %). b SP, softening point (°C). c CY, carbon yield (wt %). d C/H, atomic ratio. e TI, toluene-insoluble content (wt %). f NMPI, N-methyl-2-pyrrolidone-insoluble content (wt %). g MC, Mesophase content (vol %).

a clear relation between the structural order and the increase in size and planarity of mesogenic molecules. 13C NMR and X-ray analyses therefore provide evidence that the thermal treatment of coal-tar pitch favors the formation of planar molecules by means of the reactions mentioned above.4 3.1. Characterization of the Whole Pitches. Planar Chromatography. Thin-layer chromatography did not reveal any significant differences between the NMPsoluble fractions of the pitches during thermal treatment, these being very similar to the parent impregnating pitch. After sequential elution with acetonitrile and pyridine, three main classes of compounds were observed in daylight in all the samples: (i) immobile material retained at the origin, (ii) intermediate material, not mobile with acetonitrile, and (iii) material mobile in both solvents. Observation of the plates under UV light at 254 nm showed a fourth type of compounds also mobile in acetonitrile and pyridine. After elution with the third solvent, toluene, there were still no appreciable differences between the pitches. However, in daylight, a new group of compounds was observed near to the mobile compounds in the other two solvents. An increase in the mobility on the silica gel analytical plates indicated a decrease in the size of polynuclear aromatic rings and a decrease or absence of functional groups. Since the silica of the plates was not thermally activated and no precautions were taken to exclude moisture from the solvents or the development tanks, the size of molecules was probably the dominant factor affecting the separation.25 An examination of these results suggests that those compounds which determine the differences (in SP, CY, etc,) between the parent and the treated pitches are not detectable in the mobile or partly mobile fractions. UV-F. The structural features which may cause a shift of the UV-F spectra to longer wavelengths are alkyl and alkoxy substituents, heteroatoms containing aromatic substituent groups, the presence of linearly conformed polycyclic aromatic ring systems and formally fixed double bonds, and structures with fused fivemembered rings (with no ring carbons available for substitution). In addition, the evaluation of spectra from complex mixtures must take into account the much smaller fluorescence quantum yields from large MM materials, particularly those which contain large polynuclear aromatic ring systems which should appear at longer wavelengths. In complex mixtures, the weak fluorescence signal from large polynuclear aromatic systems tends to get swamped by the greater fluores(25) Lazaro, M. J.; Herod, A. A.; Kandiyoti, R. Fuel 1999, 78, 795801.

cence intensities of smaller polynuclear aromatic ring systems. The fractionation of samples makes it easier to obtain information for larger MM molecules.26,27 In summary, UV-F signals at longer wavelengths may be attributed to larger polynuclear aromatic systems or larger aromatic cluster sizes, as well as to heterocyclic structures and alkyl or heteroatoms substituents. Throughout the pyrolysis process no heteroatoms or alkyl substituents were introduced (NMR shows a loss of aliphatic carbons). Furthermore, the low number of hydroaromatic structures in coal-tar pitches rules out the possibility that linearly conformed aromatic ring systems are formed by dehydrogenation. Therefore, the changes in the spectra must initially be attributed to the formation of larger polynuclear ring systems at the expense of the smaller size compounds. Figure 1 shows the synchronous spectra of the NMPsoluble fraction of the parent pitch and the thermally treated pitches. Despite there being no significant differences between the spectra, possibly due to the above-mentioned swamping of lower intensity signals at longer wavelengths and the insolubility of the largest molecules, it can be observed that increasing the time of the thermal treatment results in a slight increase in the signal at longer wavelengths (more pronounced for C4 and C5) and a slight decrease at shorter wavelengths. This can be attributed to the presence of larger aromatic molecules in the soluble fraction of the more thoroughly treated samples (C4 and C5). Size Exclusion Chromatography. Figures 2a and 2b show the exclusion profiles of the parent impregnating pitch and the thermally treated pitches at 350 nm, in NMP. Two main regions separated by a valley can be distinguished in the chromatograms. The first one, at shorter elution times, corresponds to the excluded region and includes those compounds of larger molecular size which are excluded from the column’s porosity. The second region, at longer elution times, includes those compounds of smaller molecular size. The main differences between the chromatograms is the presence of a peak of small intensity in the excluded region of C1, C2, and C3, between 6.5 and 8.5 min (maximum intensity at 6.8 min, named A), which was not present in the parent pitch and disappeared after 5 h of thermal treatment (in C4 and C5). This can be explained by the formation of larger molecules which increased in size with the time of pyrolysis and became insoluble in NMP (26) Herod, A. A.; Kandiyoti, R. J. Planar Chromatogr. 1996, 9, 1624. (27) Herod, A. A.; Zhang, S. F.; Carter, D. M.; Domin, M.; Cocksedge, M. J.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1996, 10, 171.

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Mene´ ndez et al.

Figure 1. UV-F synchronous spectra of NMP-soluble fractions of the parent pitch and thermally treated pitches.

Figure 2. SEC profiles of the parent pitch and thermally treated pitches at 350 nm in NMP.



Figure 3. UV-F synchronous spectra of SEC fractions of the thermally treated pitches: (a) fraction no. 3; (b) fraction no. 2.

after 5 h. Moreover, peak no. 1 (between 8.5 and 12 min, maximum at about 9 min) disappeared in C5, a new peak appearing at about 10.5 min which was possibly present in the tail of peak 1. Molecular size transformations of pitch components due to the thermal treatment were also observed in the retained part. The two peaks present in the retained region (no. 2 and no. 3) were partially resolved. There are differences between parent pitch and C1, and between C1 and C2, as shown in Figure 2a. Apart from the continuous reduction of peak no. 3 with time, which could be due to the loss of volatiles, C1 and C2 show that the intensity of peak no. 2 increases in accordance with larger molecular size compounds, even though some of them become part of the insoluble material (see NMPI values in Table 1). The chromatograms of C3-C5 are displayed in Figure 2b. Interestingly, the peak 3/peak 2 ratio for these samples increases slightly, suggesting that the loss of large compounds is greater. This is in agreement with the above-mentioned reactivity of the components of both peaks. The remaining components of peak 3 are thermally stable while those of peak 2 (probably oligomers) react once again, giving rise to NMPI material. These results are supported by the variation in β-resin content, which decreases from C3 to C5 (from 22.8% for

C3 to 19.4 for C5). SEC results therefore clearly reveal the molecular growth resulting from the thermal treatment, despite the analytical difficulties due to the increase insolubility of the samples. UV-F of the SEC Fractions. The use of UV-F in the study of the whole sample is subject to limitations as already mentioned. Therefore, to obtain additional information on the structural changes undergone by the pitch components during thermal treatment, the five thermally treated pitches were first fractionated by SEC. Then each fraction was characterized by UV-F. As might be expected from their large molecular size, the fractions from the excluded region (peak A and peak 1) did not give any signal. Figures 3a and 3b show the spectra of fractions corresponding to peaks no. 3 and no. 2 of the treated pitches. The significant differences between the spectra of the fractions obtained from the two peaks bear out the efficiency of SEC separation by molecular size. The synchronous spectra of the fraction with the smallest size molecules (Figure 3a) are very similar to those of the whole pitches and also followed the same trend, i.e., the most intense peak was at about 380 nm. This is because in the UV-F spectrum of the parent pitch the fluorescence of the light compounds, included in peak no. 3, is dominant. These light com-

220


Figure 4. MALDI-MS spectra of SEC fraction no. 2 of the parent pitch (a) and thermally treated pitches: (b) C1, (c) C2, (d) C3, and (e) C4.

pounds remain in the treated samples because, as is well-known,28 some components of NC’s pitch such as phenanthrene, triphenylene, chrysene, and coronene are thermally stable and remain in the pitch throughout the thermal treatment. The formation of dimers (Ar-Ar) could not affect the spectra significantly and could also contribute to these findings. The increase in the time of thermal treatment produces a slight increase in intensity at higher wavelengths and a slight decrease at shorter wavelengths, which is in agreement with a molecular increase. UV-F spectra of fraction no. 2 (Figure 3b) show a significant reduction in the contribution of the smaller molecules present in fraction no. 3 and in the whole pitches. The high intensity peak at about 380 nm, is not so pronounced. High intensity peaks are observed instead at higher wavelengths (between 400 and 500 nm), together with less intense peaks, which nevertheless are still important, at the lower wavelength (about 350 nm), especially in C4. Components of these intense peaks might be expected to be present in the whole material but in lower amounts. C1, C2, and C3 are very similar, C1 showing a relatively smaller intensity at the higher wavelengths and C3 showing a slightly smaller intensity at the lower wavelength (350 nm). The pitch treated for 5 h (C4) is very different from the others, the intensity of the fluorescence being much higher at high wavelengths and also considerably smaller at the low wavelengths. These findings are very important because they indicate that during the thermal treatment of pitch, there is a change not only in the relative amounts of different molecular size compounds, but also in the (28) Yokono, T.; Miyazawa, K.; Sanada, Y.; Marsh, H. Fuel 1979, 58, 692-694.

Mene´ ndez et al.

Figure 5. MALDI-MS spectra of SEC fraction no. 3 of the parent pitch (a) and thermally treated pitches: (b) C1, (c) C2, (d) C3, and (e) C4

characteristics of the compounds in the same fraction, which increase in size with increasing time of thermal treatment. However, their size is still not large enough for them to move to the excluded region. MALDI of the SEC Fractions. The determination of MM distributions in coal-derived liquids (i.e., pitch) is rather difficult as mentioned in the Introduction because of the extreme complexity of the samples and the limitations of the techniques. GC and GC/MS normally detect aromatic compounds of molecular masses up to about 300 u and aliphatic compounds up to 500 u. In the case of MS probe, the upper mass limit for coalderived liquids is near 600 u. However, in coal-derived liquids, most of the material lies outside the MM range. The use of laser desorption mass spectrometry has considerably extended the range of molecular masses that can be detected in heavy coal-derived products, m/z 20 000 and 30 000 in pyrolysis tars and liquefaction extracts, and to 270 000 m/z in solid coals in the case of more recent MALDI-MS equipments.27,29,30 One problem with all MALDI-MS when applied to samples of wide polydispersity, is the difficulty of detecting high-mass ions in the presence of smaller molecules.31 This problem was overcome in our case by using the SEC fractions. There are also some problems associated with the detection of large MM ions in MALDI. High-mass ions are thought to produce relatively lower signals, compared to smaller ions, because the impact velocities are usually lower. Furthermore, as the mass of sample ions increased, the proportion of (29) Lazaro, M. J.; Herod, A. A.; Cocksedge, M. J.; Domin, M.; Kandiyoti, R. Fuel 1997, 76, 1225-1233. (30) Herod, A. A.; Lazaro, M. J.; Domin, M.; Islas, C. A.; Kandiyoti, R. Fuel 2000, 79, 323-337. (31) Domin, M.; Moreea, R.; Lazaro, M. J.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1997, 11, 1845-1852.



Figure 6. SEC profiles of pitch separated phases at 350 nm in NMP: (a) isotropic phases; (b) mesophases.

kinetic energy of large ions (>20 000) impacting on the collector diminished significantly.31 Processes that may explain such losses of energy include fragmentation on impact and ejection of neutral molecular fragments. Despite all the possible problems associated with this technique, MALDI is one of the few independent techniques available for providing information on molecular weight ranges and for showing the effect of the thermal treatment of pitch on the molecular size (molecular mass MM) of its components. Figures 4 and 5 show the MALDI-MS spectra of the SEC fractions (2 and 3) of parent and treated pitches. The spectrum of fractions 2 and 3 of the parent pitch show a peak of intensity between 900 and 1200 u and both sets of spectra appear to reach the baseline by about 9000 u, although there is some evidence of masses up to 19 000 u. In general terms, for all the treated pitches, the peak of maximum intensity is between 1000 and 2000 in both fractions but the fall in intensity with increasing time appears to extend from 12 000 for C1 and C2 to 30 000 for C3 and C4. However, a detailed study of the results shows that the absolute maximum of intensity of fraction 2

shifts to higher values with increasing time from 1198 for C1 (2 h) to 2055 for C3 (4 h). In the case of C4 (5 h) the maximum of fraction 2 appears at the lower value of m/z 926. The same occurs with fraction 3, where the maximum intensity shifts from 966 for C1 to 2055 for C3. Again C4 shows a lower value of 917. There are three points to be considered: (i) the increase in MM of the fraction from peak 3 to the fraction from peak 2 in each of the products, (ii) the increase in MM with time up to 4 h (C3), and (iii) the decrease in MM in both fractions after 5 h of treatment (C4). The first point confirms separation takes place in accordance with molecular size in the SEC system. The second point confirms the polymerization of the sample as a result of the time of thermal treatment, and the third (in agreement with SEC results) the presence of lower MM compounds in the soluble fraction of those pitches with a high mesophase content. These compounds are present in the whole series but are accompanied by those of larger molecular size which become NMP insoluble. Even more important, however, is the fact that during polymerization not only is there a movement of com-

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Figure 7. SEC profiles of pitch separated phases at 450 nm in NMP: (a) isotropic phases; (b) mesophases.

pounds from one fraction to another as a result of their increase in size, but also the compounds left behind increase in size. 3.2. Characterization of Separated Phases. There are significant differences between the SEC chromatograms of the single phases (isotropic and mesophase) and those of the whole pitches, and between the chromatograms of the isotropic phases and mesophases themselves as shown by Figure 6. The excluded region in the isotropic phases shows two well-resolved peaks between 8.5 and 12 min, while just one was observed in the case of the whole pitches. In the region of the column where molecules are excluded from the porosity, separation depends on the facility of penetration between the column packing particles. Resolution will depend on shape alone. In the retained region, the peak including compounds of smaller size (peak no. 3, 19.523 min), becomes less important relative to peak no. 2 (15-19.5 min), the effect being more pronounced with the thoroughness of treatment. This suggests that the compounds of apparently larger size in the retained region of the whole pitch form a part of the isotropic phase.

The opposite was the case found in the retained region of the mesophase fractions, where peak no. 3 is similar to or even higher than no. 2. In the excluded region of the mesophases, peak no. 1 is resolved in two peaks, as in the case of the isotropic phases, but the most spectacular finding is the presence of a prominent peak (A′) at about 7.5 min (slightly shifted to a longer retention time with respect to peak A), corresponding to the largest-size compounds. This indicates, as can be calculated from data in Tables 1 and 2, that in the mesophase, the fraction soluble in NMP is not only formed by the isotropic part that remains after filtration, but also by larger molecular size compounds, although it is possible that the two peaks between 8.5 and 12 min belong to the isotropic material that remains behind. The possibility that peak A′ may be due to molecular associations was ruled out because the same elution profiles were obtained with highly diluted samples. Peak no. 3 to some extent belongs to the mesophases and corresponds to smaller size compounds of pitch retained or trapped in the mesophase (as proved by thermogravimetric analysis21). Finally the significant


signal of the peaks at the front of the excluded region must correspond to very large molecular size compounds as demonstrated not only by their short elution time but also by the strong-intensity UV-absorbance signal at 450 nm (Figure 7). An explanation for why compounds in peak no. 2 of the isotropic single phases have apparently larger sizes than those of the single mesophases could be their different molecular structure. While compounds in mesophases are planar and more insoluble, those of the isotropic phases could be tridimensional oligomers and consequently more soluble. It needs to be borne in mind that the formation of oligomers which are not initially planar takes place in the initial stages of the thermal polymerization. These molecules become planar with the formation of new intramolecular bonds and ring closure.4 A point of considerable importance, which may help to comprehend and explain the spectacular chromatograms in the exclusion region of the mesophases, is the improvement in solvent accessibility and sample solubility with ultrasonics. During the preparation of the NMP solutions in the experimental part, ultrasonics was used to favor the solubilization of samples. The time chosen was 30 min. The use of longer times did not make any difference to the intensity of the peaks of the SEC chromatograms in the case of the whole pitches and the single isotropic phases. However, things were rather different in the case of single mesophases, the intensity of A′ increasing with time up to 1 h in ultrasonics, stabilizing for longer times. 4. Conclusions The thermal treatment of pitch resulted in the removal of light compounds and a progressive dehydrogenative polymerization, with the formation of larger aromatic molecules, as confirmed by the NMR and UV-F results. Planar chromatography of the whole samples suggests that those compounds which determine the


differences in the SP, CY, etc., of the parent and the treated pitches are not detectable in the mobile or partly mobile fractions. UV-F spectroscopy of the SEC fractions indicates that during the thermal treatment of pitch, there is a change not only in the relative amounts of compounds of different molecular size, but also in the characteristics of the compounds present in the same fraction. Although the size of these compounds increases with increasing time of thermal treatment, they are still not large enough to move to the excluded region. The polymerization of the samples as a result of thermal treatment was also confirmed by the MALDI studies performed on the SEC fractions, which showed the movement of compounds to fractions of higher MM. The increase in size of the compounds remaining behind in each fraction was also evident. Moreover, the results also corroborated the presence of lower MM compounds in the soluble fraction of those pitches with a high mesophase content. The results obtained from the study of the separated isotropic phases and mesophases suggest that the components of these phases have a different molecular structure, the polymerization products changing from nonplanar oligomers in the isotropic phase to more condensed planar molecules in the mesophase. Acknowledgment. The authors thank the University of London Intercollegiate Research Service (ULIRS) for providing NMR facilities (Kings College and University College) and MALDI mass spectrometry (School of Pharmacy). I.S. thanks the European Commission for grants (Marie Curie Research Grant, Nonnuclear Energy and Energy, Environment and Sustainable Development programs). R.M. thanks the Royal Society and CSIC for granting her stay at ICSTM. C.B. thanks the EU for a Marie Curie Fellowship (Contract HPMF-CT-1999-00233). EF000191R

On the Chemical Composition of Thermally Treated Coal-Tar Pitches

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